Menu can be set to fullscreen via XFWM keyboard shortcut
The XFWM4 keyboard shortcut to fullscreen a window (Alt+F11 by default) applies to the Whiskermenu and causes it to maximize outside the bounds of the screen. This forces the user to manually edit the config file if they want to resize it again.
I'll see if I can do some more research on how to prevent this myself, but it seems that you can only really disable resizing windows all together with GTK, rather than just the action of fullscreening them via the window manager.